Conventionally, the method of spraying refractory material has been used only for lining the inside of small constructions such as chimneys and ducts. However, recently, the method has been used for the installation of industrial furnaces, in which the whole of the lining is formed by spraying monolithic refractory material.
The spraying of the material is generally conducted at room temperature, but in the course of recent repairing of industrial and metallurgical furnaces the tendency has been to spray refractory material while the furnace is still hot, as it is considered unfavorable and a loss of time to stop and cool the furnace for repairing merely a partly worn portion.
Spraying of monolithic refractory material has two kinds of processes, i.e. a dry process and a wet process. In the dry process, the powdered material is transmitted from the container to the nozzle by compressed air in a dry state through a hose, then mixed with water and sprayed. In the wet process, the powdered material is mixed with water in the container and sent to the nozzle in a muddy state, and sprayed by the compressed air.
In either process, however, the refractory material is sprayed through the nozzle which is spaced apart from an opposed work surface, and the material is forced to adhere to the surface by collision. As a result, some of the material may rebound or be carried away by the air stream, and such causes loss of the material. The loss becomes particularly large when material is applied to a vertical wall or a ceiling, and sometimes amounts to about 10% of the material used. Further, once the material is hydrated, it cannot be reused, so that, this kind of loss is considered to be a limitation to the development of the spray method.
The conventional spray method also has the following disadvantages.
(a) Rebounding material scatters around as dust, and injures the work environment.
(b) As the material is sprayed on the work surface layer by layer to form a lining, the lining easily peels off when it exceeds a certain thickness.
This limit of the thickness is about 100-120 mm. As a matter of fact, use of the spray method is restricted to merely repair of locally worn portions, because of this thickness limit.